Double metal cyanide (DMC) complex compounds are well known catalysts for epoxide polymerization. The catalysts are highly active, and give polyether polyols that have low unsaturation compared with similar polyols made using basic (e.g., KOH) catalysts. Polyols with low unsaturation are desirable because they give polyurethanes with an excellent balance of physical and mechanical properties.
DMC catalysts are made by reacting aqueous solutions of metal salts and metal cyanide salts to form a precipitate of the DMC compound. A low molecular weight organic complexing agent, typically an ether or an alcohol, is included in the preparation. The complexing agent is incorporated into the catalyst structure, and is required for an active catalyst. In a typical catalyst preparation, the precipitated DMC compound is washed several times with aqueous solutions containing the organic complexing agent, and is isolated by centrifugation or filtration. Finally, the catalyst is dried to a solid cake, usually in a vacuum oven. The dried catalyst is then crushed to give a free-flowing powder. The powder form of catalyst is commonly used for polymerizing epoxides. U.S. Pat. No. 3,829,505 and Jap. Pat. Appl. Kokai No. 4-145123 illustrate typical catalyst preparations; each includes details of how to dry and crush the catalyst before use.
Van der Hulst et al. (U.S. Pat. No. 4,477,589) teaches the preparation of powder DMC catalysts. In addition, this reference teaches that suspensions of DMC catalysts in propoxylated glycerin starter polyols can be used, thereby eliminating the need to isolate a powder catalyst. In making a suspension, a DMC catalyst is precipitated in the usual way. The aqueous catalyst mixture is treated with an organic complexing agent, and the suspension of catalyst, water, and complexing agent is combined with propoxylated glycerin. This mixture is stripped to remove water and excess organic complexing agent, leaving a suspension of DMC catalyst in propoxylated glycerin. The suspension, which contains about 3 to 5 wt. % of DMC catalyst, is then used as a catalyst in the reaction of additional starter polyol and propylene oxide to make a polyol. Thus, the reference teaches to use as a catalyst either a powder DMC catalyst or a dilute suspension of DMC compound in propoxylated glycerin. Despite the apparent advantages of the suspension approach, powder catalysts have been more widely used.
Powder DMC catalysts having exceptional activity for epoxide polymerization are now known in the art. See, for example, U.S. Pat. No. 5,470,813. However, even the best powder DMC catalysts have some disadvantages. First, drying the catalyst after isolation is time-consuming and requires a vacuum oven. Drying large quantities of catalyst is especially taxing. Second, the dried catalyst must be crushed to produce a powder. This step requires an expensive crusher, pulverizer, or mill. Both steps are costly in terms of capital costs, labor, and time requirements, and they add significantly to the overall cost of production.
The drying and crushing steps can adversely affect catalyst quality and performance. Excessive heating during the drying stage can cause catalyst degradation and reduced activity. Catalyst heat-up due to friction during crushing of the catalyst can also adversely impact catalyst performance. Variations in how crushing and drying are done from batch to batch can result in inconsistent catalyst performance and variations in polyol quality.
Improved double metal cyanide catalysts are needed. Preferred catalysts will have high activity, as those described in U.S. Pat. No. 5,470,813. Particularly needed are catalysts that can be made without drying or crushing steps, which add significantly to the overall cost of production. An especially valuable catalyst could be made with improved batch-to-batch consistency, and would enhance the quality of polyether polyols made using the catalyst.